Soften the leeks in the butter. Season well. Peel the potatoes and grate them. Wash them well in a sieve and dry in a clean tea towel.
Heat the goose fat and using half the potato, press four flat little rounds of potato into the pan. Put a layer of leeks on each round, then cover with a further layer of potatoes. Cook over a fairly high heat until golden on each side of the cakes.